1. Field of the Invention
The present invention lies in the field of processes for using rotating biological contactors in the biochemical treatment of wastewater for the removal of carbonaceous and nitrogenous pollutants.
2. Prior Art
In general, prior art biochemical wastewater treatment processes comprise two or three identifiable phases. An initial phase typically consists of the removal of grit and screenable material. In this phase, floatable and settable matter is removed through mechanical means. In a second phase, solutes in wastewater are converted biologically to microbial cells which are themselves floatable or settleable. These cells are subsequently removed mechanically in what may be considered to be a part of the secondary phase. If the secondary or biological treatment process is unable to meet required water quality standards, a so-called tertiary wastewater treatment process may be required for a given wastewater treatment system. Although such a tertiary process treatment may also involve biological conversion, it typically consists of physio-chemical processes in combination with stringent mechanical means.
The secondary (biological conversion) phase provides a means of bringing microbial organisms in the form of activated flocs or attached slimes into contact with the wastewater which provides the organisms with the required nutrients for proliferation. Some organisms are retained to continue the process, but the majority are subsequently separated from the resulting treated wastewater.
In the prior art, the rotating biological contactor (RBC) has gained attention for use in the secondary phase because of its relative simplicity in design and cost effectiveness. The RBC provides a biological contactor which is a medium fixed to a shaft that rotates and which is so positioned in an aerobic system as to cause the medium to be partically submerged in the wastewater. The medium (or contactor) provides a large surface area for microbial attachment. As the shaft rotates, the medium is alternately immersed and emersed in the wastewater, thereby providing exposure to the soluble nutrients and to atmospheric oxygen alternatively. Through the action of liquid sheer forces, or from a sequence of starvation and/or senescence of the innermost cells in thick slimes, excess biomass is regularly separated from these medium. In addition, through entrainment of air in the turbulence created as the medium is rotated through the fluid, the RBC imparts a degree of aeration which is important in keeping the microbial community viable.
Multistage RBC configurations have heretofore been employed in the secondary treatment phase and have been shown to provide generally better performance than a single stage reactor of equal volume. Multistage wastewater treatment systems utilizing partially submerged rotating biological contactors are shown and described, for example, in Welch, U.S. Pat. No. 3,557,954; Torpey, U.S. Pat. No. Re. 29,970; Hankes and Parker U.S. Pat. No. 4,608,162; and Smith et al "Proceedings: First National Symposium/Workshop on Rotating Biological Contactor Technology", Champion, PA, Feb. 4-6, 1980, sponsored by University of Pittsburg, U.S. EPA Municipal Environmental Research Laboratory, Cincinnati, Ohio, and the U.S. Army Construction Engineering Research Laboratory, Champaign, lllinois, June, 1980 (Vol. I and II).
As attention to water quality demands increasingly stringent discharge standards, the need for economical systems for meeting such standards becomes increasingly apparent.
Previously it has been proposed in the field of RBC process to employ recycling. For example, Wittmann et al U.S. Pat. No. 4,563,282 describe recirculation of effluent directly from an RBC back to the influent thereto. The returned wastewater is distributed to the down-rotation side of their RBC which is provided with cups and pockets, thereby "imparting additional rotation torque to the rotating biological contactor. Additional benefits are achieved through spreading the distribution of the circulating flow over a greater rotating biological contactor surface area." (see Wittmann et al '282 at column 4, lines 37-41), however nothing Wittmann et al teaches or suggests the use of clarified effluent for recirculation at high rate. Wittmann et al teach RBC use in combination with equipment used in an activated sludge process wherein suspended solids concentrations typically are greater than about 1500 milligrams/liter (mg/l) and SBOD.sub.5 can be less than about 15 mg/l, depending on the sampling point.
For another example, the U.S. Environmental Protection Agency has recognized the use of recirculation of secondary clarifier effluent for purposes of providing flexibility in process operation; see, for example, the EPA brochure entitled "Rotating Biological Contactors (RBC's)--Checklist For A Trouble - Free Facility". However, such teachings do not suggest the use of recirculation as an integral and normal part of a wastewater treatment practice.
The wastewater treatment art needs new and improved process utilizing the RBC which can be employed effectively for the removal of both carbonaceous and nitrogenous material and which are characterized by a capacity to handle significant surges in contaminate levels in incoming wastewater.